1. Field of the Invention
The invention relates to a shadow mask and to a process for the manufacture of a shadow mask made of an iron/nickel alloy. The shadow mask of the invention is particularly suited for a color display cathode-ray tube.
2. Discussion of the Background
A color display cathode-ray tube generally comprises an envelope having a display window made of glass, including a display screen on which red, green and blue luminophores are placed. A shadow mask, perforated with a very large number of small holes, is mounted in the tube, opposite the display screen and at a short distance from it. When the tube is operating, three electron beams are generated inside the tube by three electron guns, the electron beams passing through the holes in the shadow mask and bombarding the phosphorescent areas.
The relative positions of the holes and of the luminophores are such that each electron beam bombards the phosphorescent areas corresponding to a particular color in order to form a picture. However, a significant part of the electrons is intercepted by the shadow mask and the kinetic energy of these electrons is converted into heat which raises the temperature of the shadow mask. The thermal expansion of the shadow mask, generated by this temperature rise, can cause local distortion of the shadow mask, which causes a disturbance in the placing of the holes relative to the associated luminophores. This results in errors in the colors in the picture made, and these errors are more significant the flatter the shadow mask, which is increasingly the case in current generations of cathode-ray display tubes.
It is well known that such problems, caused by thermal effects, can be avoided by manufacturing shadow masks from a material having a very low coefficient of expansion such as, for example, an iron/nickel alloy containing approximately 36% nickel. However, the high level of the mechanical properties and the difficulties in rolling such materials limit their use for this application.
It is known, from United States Patent U.S. Pat. No. 4,685,321 (EP-A 179,506), to firstly subject a foil made of such a material, intended for the manufacture of a shadow mask, to a heat treatment in order to reduce its 0.2% yield stress at room temperature and to then perform shaping above room temperature so as to further reduce its 0.2% yield stress. The iron/nickel alloy used in this process has a coefficient of linear expansion of between 1.times.10.sup.-6 K.sup.-1 and 1.5.times.10.sup.-6 K.sup.-1. A lower coefficient of expansion can be obtained by replacing part of the nickel with cobalt in amounts of between 2% and 12% by weight. However, the substitution of nickel by cobalt has many drawbacks. On the one hand, cobalt is a very expensive element and, on the other hand, cobalt contaminates the chemical etching reagents used for drilling the holes in the shadow mask by chemical etching.